1. Field of the Invention
The present invention relates generally to the field of electrical current sensing devices such as those used in programmable logic controllers and other environments. More particularly, the invention relates to a method for sensing current through the use of a high resolution electromechanical device which can be formed through micro-machining and similar techniques and incorporated into control, monitoring and similar circuits.
2. Description of the Related Art
A wide range of applications exist for devices capable of accurately sensing electrical current. In certain applications, current is sensed for simple readout, such as on a metered scale or digital display. A considerable number of applications, however, require current to be sensed for use in regulation of power or as feedback for the control of machines, circuits, and processes.
In the industrial environment, for example, feedback devices and actuators typically operate within specified current and voltage ranges. Feedback from sensors may assume values within the acceptable range, with the values reflecting physical parameters of a controlled system. One such application is in programmable logic controllers (PLC's) in which a 4-20 ma current range is typically provided for control and feedback. To enhance the performance of systems incorporating these devices, it is often desirable to obtain very high resolution current sensing in a manner which avoids unnecessary power drain from the associated circuitry or system.
Digital current sensors presently in use in applications such as PLC's suffer from several drawbacks. In certain known 4-20 ma current sensors, for example, isolation from perturbations which may be caused by external circuitry is achieved by electrically "floating" an analog-to-digital converter and its associated electronics. This involves a floating power supply circuit which uses a DC-to-DC converter which receives an input from a 5 volt DC power supply, converts it to an AC signal, transforms the AC signal for isolation, and reconverts the AC signal into DC power. The isolated electronics then communicate with non-isolated electronics through a pair of digital opto-isolators. These isolators are then coupled to a clock and to an output which returns digital values in serial form. However, opto-isolators used in such devices are, in general, unsuitable for use in an analog fashion for resolutions higher than 6-8 bits, due to their temperature dependency and drift. Moreover, to reduce costs in such sensor circuitry, analog inputs for a single module are generally not isolated from one another so that they can share a same floating power supply.
An important drawback in present state-of-the-art current sensors is their unit cost. Even in applications where a single current sensor suffices for feedback or control, sensors of the type described above can add significantly to the overall system price. Moreover, in many PLC applications, it is desirable to provide current sensors on many or all outputs of a PLC to monitor output current for system control, as well as for diagnostic monitoring, such as for output protection. As noted above, similar high resolution, low cost requirements exist for sensing on input channels of PLC's and other devices.
Still further drawbacks in existing technologies include inconsistencies in device-to-device performance, in compatibility with microelectronics, parasitic losses, and the physical dimensions of the current sensor and associated circuitry. These and the foregoing drawbacks can further lead to problems with energy dissipation, heating and other thermodynamic and performance problems.
There is a need, therefore, for an improved technique for providing high resolution current sensing at a relatively low cost for both the sensing device and its associated circuit. There is, at present, a particular need for a current sensing technique which offers enhanced performance in a reduced package size, facilitating both manufacturing and incorporation into electrical and microelectronic devices such as PLC's.